1. Field of the Invention
The present invention relates to the production of porous polymers. More particularly, it relates to a method of producing porous polymers having polar groups, an average pore diameter of about 40 A or more, a high pore volume and high mechanical strength.
2. Description of the Prior Art
Porous resins have a wide range of use as ion exchange resins of high efficiency, adsorbents, stationary phases for chromatography, carriers for catalysts and immobilized enzymes, etc., due to their high surface area, fine pore structure and high rate of mass transfer in the resins. Especially with regard to ion exchange resins having high porosity, there are various advantages such as a high rate of ion exchange with an increase in the ion diffusion velocity in resins, small swellability and shinkability in contact with a liquid, high regeneration efficiency and high resistance to organic materials.
Many prior art references teaching various preparation methods for porous resins. For example, British Pat. Nos. 932,125 and 932,126 describe a process for preparing a porous cross-linked copolymer by effecting suspension polymerization of a monomer mixture in the presence of a precipitant which is a liquid substantially insoluble in water (a) which dissolves the monomer mixture and (b) which does not swell the product copolymer in an amount sufficient to separate the product copolymer from the monomer phase. In this method, phase separation takes place when about 40 to 50 percent by weight, based on the total weight of the monomers, of a precipitant is employed, and the pore volume becomes less than about 50 percent based on the resin as such since the pore volume basically depends upon the relative amount of the precipitant. Furthermore, when the preparation of porous copolymers having a high pore volume is attempted, phase separation takes place so much that the mechanical strength of the product copolymers diminishes.
Japanese Patent Laid Open 71790/1973 describes a method of preparing a macroreticular cross-linked copolymer by copolymerizing a vinyl nitrogen heterocyclic monomer such as a vinylpyridine monomer with a polyvinyl aromatic hydrocarbon such as divinylbenzene in the presence of a phase extender (or an precipitant) in an amount sufficient to cause phase separation of the cross-linked copolymer. In this method, the phase extender may be employed in an amount of about 30 to 150 percent by weight based on the total weight of the monomers employed.
U.S. Pat. No. Re. 27,026 teaches a method of preparing a porous polymer having a predetermined porosity by polymerizing a monomer mixture of (a) at least one monomer selected from styrene type monomers, acrylates or methacrylates and vinyl carboxylates and (b) a cross-linking agent such as divinylbenzene, diethylene glycol dimethacrylate, etc., in the presence of a solvent mixture comprising: (a) at least one solvent and (b) at least one non-solvent, said solvent mixture having a solubility parameter chosen within the range .delta. = .delta..sub.0 .+-. 0.8, where .delta. is the solubility parameter of the polymer and .delta..sub.0 is the solubility parameter of the polymer, to control the average pore size of the polymer.
British Pat. No. 1,274,361 describes a process for the production of porous resins by copolymerizing at least one monovinyl compound with at least one polyvinyl compound in the presence of: (a) at least one non-swelling adjuvant which dissolves the monomers but neither dissolves nor swells the polymers and (b) at least one swelling adjuvant which dissolves the monomers but only swells the polymer without dissolving them.
German Patent Laid Open 2121448 describes a suspension copolymerization in water of an ethylenically unsaturated monomer with a copolymerizable cross linking monomer in the presence of a mixed organic liquid composed of: (a) a component which solvates the copolymer such as an aromatic hydrocarbon and (b) a precipitating component for the copolymer such as an aliphatic hydrocarbon.
According to the latter three methods, porous resins having a microporous structure and a high pore volume can be produced by carrying out the polymerization in a large amount of appropriately chosen organic liquids. These methods, however, can be employed only when the polarities of the monomers employed are similar each other. For example, according to U.S. Pat. No. Re. 27,026 the solubility parameter for the polymers to be prepared is in the range of from about 8.8 to about 9.7 (cal/ml).sup.1/2. However, when a part of the monomers employed is a polar monomer, the polarity of the copolymer product varies to a great extent, even if the amount of the polar monomer is comparatively small, and the determination of a suitable solvent system is often difficult.